1. Field of the Invention
The present invention relates to a fuel assembly in a pressurized water reactor, and in particular, to a foreign matter filter serving as a protection means against foreign matter for preventing intrusion of foreign matter into a fuel effective portion in a coolant.
2. Description of the Related Art
FIG. 25 is a perspective view drawing showing a fuel assembly generally used in a pressurized water reactor. FIG. 26 is a sectional view showing a typical nuclear reactor.
A fuel assembly is composed of an upper nozzle 1 and a lower nozzle 2 which are spaced apart from each other, a plurality of grids 3 being spaced at a predetermined interval and having lattice spaces formed by straps, control rod guide pipes (guide thimbles) 4 inserted into the lattice spaces at a predetermined interval, fastened by fastening portions of the grids 3, and having upper and lower end portions respectively connected to the upper nozzle 1 and the lower nozzle 2, and a large number of fuel rods 5 elastically supported by support portions of the grids 3. In the fuel rods 5, fuel pellets and springs are hermetically sealed by covering tubes and upper and lower end taps. A tubular insert 10 with an end tap is connected to the lowermost grid, and the guide thimbles 4 are inserted into the insert, with the guide thimbles 4 and the lower nozzle 2 being connected to each other. The portions of the lattice spaces at the positions of the guide thimbles are referred to as thimble cells, and the portions of the lattice spaces at the positions of the fuel rods are referred to as fuel rod cells.
The nozzle 2 is arranged directly above an opening in the lower portion of the nuclear reactor, coolant flowing from here to the portion of the reactor above the reactor core. In view of this, the lower nozzle 2 has, as shown in FIGS. 27A and 27B, a large number of flow passage holes 6 for allowing the coolant to pass through.
During production, installation, and repair of piping and cooling equipment, various kinds of foreign matter including splits and fragments, such as metal particles, chips, and shavings, can get into the coolant, and it is to be assumed that these fragments of foreign matter are held between the walls in the cells of the grids 3 and the fuel rods 5 after flowing into the interior of the fuel assembly. Movement and vibration of the held foreign matter due to the coolant flow lead to abrasion of the fuel rods 5 and removal of the covering tubes (piercing of the covering tubes). This leads to harmful effects, such as corrosion of the fuel rods 5 and the impossibility of retaining radioactive fission gas products in the fuel rods 5 (fuel leakage).
To solve this problem, there is adopted, as a first prevention means against foreign matter, an anti-foreign-matter lower nozzle (hereinafter referred to as DFBN) to minimize the amount of fragments of foreign matter in the coolant flowing into the fuel assembly, reducing the diameter of the flow passage holes through which the coolant flows, and further, as a second prevention means, an elongated solid lower end tap is put in the lowermost grid, utilizing the gap between the lowermost grid and the elongated fuel rod lower end tap portion. That is, it is assumed that foreign matter having passed through the lowermost grid constituting the second prevention means, also passes through the grid situated on the downstream side, and does not bring about fuel leakage.
However, from now on, the fuel assembly will be improved toward higher burn-up, and, with that, the internal pressure of the fuel rods will become severer, so that it is desirable that the fuel rod lower end taps be shortened in order to restrain the internal pressure, thereby increasing the plenum volume.
As a result of irradiation growth of the fuel rods, the fuel rods are displaced downwardly, so that the covering tubes get out of the lowermost grid; thus, the shortening of the fuel rod lower end taps involves a problem in that the trapping of foreign matter fragments by the gap between the fuel rod lower end tap portions (solid portions) and the lowermost grid, which is the second anti-foreign-matter means, cannot be sustained over the entire service life of the fuel assembly.